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I'm working on a red instant and sorceries deck featuring Charmbreaker Devils. As I understand it, this ability:

At the beginning of your upkeep, return an instant or sorcery card at random from your graveyard to your hand.

is a triggered ability. So I should be able to use Strionic Resonator to copy it. How exactly does copying it work? Is there a chance that the same card in my graveyard is chosen twice, or will I always get two instants or sorceries back, as long as there are two to start with?

First:Charmbreaker Devils will put its triggered ability on the stack.

Second: Before that resolves, you may activate Strionic Resonator. One of its rulings states:

7/1/2013: Strionic Resonator targets a triggered ability that has triggered and is on the stack.

So you copy the triggered ability currently on the stack, and put the copy on the stack above the original.

Third: You then do nothing, and let these resolve.

Fourth: The copy resolves first. You return a random instant or sorcery card from your graveyard to your hand.

Fifth: The original resolves. You return a random instant or sorcery card from your graveyard to your hand. This cannot possibly target the card you just returned, since it's no longer in your graveyard to return.